Wireless communication technology is evolving at an ever growing pace in order to meet the demanding performance characteristics of new mobile wireless communication devices. It is desired that these new mobile wireless communications devices are able to transmit data with a minimum amount of signal distortion.
There are two common types of transmitters employed in today's wireless infrastructures. In some transmitters an information signal (e.g. audio, video, etc.) modulates a radio frequency (RF) signal. This is known as direct modulation, and these direct modulation transmitters are relatively simple.
Other, more complicated transmitters are called superheterodyne transmitters. In a superheterodyne transmitter, the information signal first modulates an intermediate frequency signal. After stages for filtering and amplification, the intermediate frequency signal is converted to a RF signal by a frequency mixing stage. These superheterodyne transmitters are more complex than direct modulation transmitters, although they do provide numerous advantages.
When the intermediate frequency signal is converted to the RF frequency through a mixer, a variety of undesirable frequencies in addition to the desired frequencies are generated. The undesirable frequencies are based upon both the intermediate frequency and the information signal Common undesired signals include local oscillator feed through and the IF image frequency response. Subsequent stages, including filters, are used to remove these undesirable frequencies. When a given device is capable of transmitting at multiple frequencies, advanced filtering stages can be utilized to filter out the different undesirable frequencies corresponding to which transmit frequency is currently being employed.
For example, U.S. Pat. Pub. 2008/0287089 to Alles discloses an input filter for a superheterodyne receiver for image frequency suppression. The input filter includes a first filter circuit with bandpass characteristics and a center frequency. The first filter circuit has a varactor diode and a first filter inductor that are connected in parallel and form a parallel-resonant circuit, and the center frequency of the first filter circuit can be set by application of a control voltage to the varactor diode. The receiver also includes a second filter circuit with band stop characteristics that includes a varactor diode and a second filter inductor being connected in series and forming a series-resonant circuit.
Similarly, U.S. Pat. No. 7,221,924 to Zheng et al. discloses a superheterodyne receiver including a notch filter. The notch filter includes a varactor. Tuning of the varactor tunes the notch of the filter.
The filters in the above references may not provide the desired performance, because varactors may behave in a highly nonlinear fashion at higher frequencies, or may not be able to handle a desired amount of power, due to the overall small size of varactors. Consequently, new filter designs for electronic devices are required.